mjhjmtu

I believe it's as simple as "nonparallel". A synonym for this is "oblique" ("slanting or inclined in direction or course or position--neither parallel nor perpendicular nor right-angled").
https://www.vocabulary.com/dictionary/nonparallel

The opposite of parallel is right, orthogonal, normal or perpendicular. But these towers are not the opposite of parallel, they're simply not parallel. So you could just say "not parallel". You could also say "at an angle to each other". Technically, parallel lines are at angle of 0, and 0 is a number, but people will know what you mean. You could also say they "diverge" or "are divergent". Here, it would be implicit that they diverge as one goes up.

Use of a descriptive word along with "parallel" seems appropriate here - adverbs like almost, approximately, virtually, or visually.

The nature of the construction and the deviance from parallel is so small that using a completely different word would seem to convey greater difference than is actually present. The two bridge towers are almost (but not quite) parallel, differing, top distance vs. base distance, by a small but significant 41.26 mm. They are:

"virtually" parallel. or "approximately" parallel

or

They "deviate" from being truly parallel.

Example: Though virtually parallel, they actually deviate from parallel by 41.26
mm at top to correct for the curvature of the earth.

https://www.thefreedictionary.com/virtually

https://www.thefreedictionary.com/deviate

I think you could go with askew:

out of line : at an angle
_{from m-w.com}

They are divergent or diverging

Drawing apart from a common point; diverging. (Free Dictionary #1)

"Angled". But, uh, in your picture it is clearly the pincushion distortion of the lens that is doing a much more thorough job of sabotaging parallelism than Earth curvature (the radius is something like 4000mi after all). And after pincushion distortion, there is perspective distortion. And of course you'd not make the pillars of a suspended bridge vertical anyway but angle them outwards so that the combined load of their weight and the rope tension will point towards their foundation.

Divergent seems sensible to me

there are many good answers here for what you want, but technically none of them are the opposite of parallel. that would be perpendicular or orthogonal

If the word "parallel" is used in the computational process, or some other process task, you can use "serial" as oposit of "parallel".

• 
  

  Example 1:

  
  
  

  My computer can process 8 tasks in parallel.

  
  

The oposite

My computer has an serial processor.

• 
  

  Example 2:

  
  
  

  My team can perform 10 process in parallel (i.e. in the same time)

  
  

The oposite

My team can perform only processes serially

Although it doesn't quite fit into your sample sentence, the word radial describes the tilted arrangement of the bridge towers:

Of or arranged like rays or the radii of a circle; diverging in lines from a common centre.
_{Oxford Dictionaries}

In electricity the opposite of "parallel" is "series" relative to connections. It would also apply to these towers as they appear in series along the bridge roadway.

"Converging" would the most apt descriptor for "not parallel" in my opinion, as geometrically speaking any lines which are not parallel can accurately be said to converge or diverge.